do i still have to repack the bearing by hand or can i just fill it with grease through the grease fitting?

It is best to repack your bearings at least once a year. The bearing buddy was designed to allow you to place grease in the hub to replace what is lost/broke down during normal wear and tear from driving, and also to help keep water from entering the hub assembly. That is part of the reason that the BB has a spring inside. It will keep the interior pressure higher than the outside pressure applied when the trailor is submerged.

Some hubs also have a grease fitting on the back of the hub. The rear mounted grease fitting and the bearing buddy is by far the best design that I know of. It will get the grease to the back bearing without forcing grease out of the rear seal.

I always pack my bearings and fill the hubs with grease.  Good seals will trap air and if you don't pack the bearings you may end up running the inner bearing dry.  Just be sure you use the same tube grease you plan to use in your grease gun, and don't use a thick wheel bearing grease.